Topical thermography, wherein micro-encapsulated liquid crystals are used for medical diagnostic purposes, has been severely inhibited because of poor persistence of the thermal image created in the liquid crystals. When a conventional liquid crystal sheet is applied to a body surface, the display of color resulting from the variations in temperature along the body surface must be observed or photographed promptly then and there, because once the sheet is removed from the body surface, the heat loss from the liquid crystals causes the colors to change and produce a false reading. Not only does this impose undesired requirements for camera set-ups and for undue haste where deliberation may be called for, but it makes it virtually impossible to use topical thermography for observing temperature variations along surfaces of body cavities.